You may have heard, from the internet or from someone making idle conversation at a cocktail party, that mushrooms are more like humans than they are like plants. What does that even mean? We'll work it out for you.

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Occasionally, when people assert scientific facts in everyday life, I get annoyed. A prime example is when people talk about how strawberries, blackberries, and raspberries aren't really berries, since berries have their seeds on the inside and strawberries line themselves with their seeds. As far as I'm concerned, the word "strawberry" comes down from Old English a thousand years ago, and was pronounced "berry" by the 1400s, while the Linnaean System of classification has only been around since the seventeen hundreds. If there's a etymological problem, it's up to Linnaeans to figure out a berry definition that includes the most commonly-known berries, or if the don't, to come up with a new word entirely. If they don't care to, at least they can keep their technicalities away from my brain, instead of deciding that they get to re-classify all berries.

I got the same impression when I started hearing that mushrooms were closer to humans than they were to plants. Obviously, behaviorally, they aren't. They don't move. They don't reproduce sexually. They don't squeak when you poke them. This is why they, up until recently, were pushed over to the plant side of most compendia of living things. However, unlike the Linnaeus-Come-Latelies, there are actual genetic and biological reasons for us to suddenly look at mushrooms as our somewhat close relatives.

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There are genetic and biological reasons for us to be more closely related to nearly everything than we previously thought. It was originally thought that land-based life forms came from the many different types of algae floating around the sea. Now it looks like first life hopped to fresh water and the original land-based life came from only one type of algae in that fresh water. This means that we're more closely related to anything on land than water-based algae - even the stuff that's evolved symbiotically with us and is swimming around our guts right now.

That's significant because originally people thought that fungi came from the supposed multitude of algae ancestors. Somehow, fungi evolved from one of the many algae that hopped onto land - specifically a type of algae that lost its chlorophyll. So it was one of the many things that slimed its way on shore, something totally separate from the slimy thing that eventually became humans. The first thing that pushed us together with the toppings on our so-called vegetarian pizzas was that one algal common ancestor.

But how do we know that that commmon ancestor budded out plants and fungi so far apart that the fungi are closer to us than to plants? We take a look at genetic similarity, and how that kicks over into physiology. There was a marked lack of chlorophyll in the near history of both animals and fungi. We both took a step away from photosynthesis before we started becoming what we are. Fungal cell walls are made of chitin, the same thing that makes up insect's outer carapaces, but is found nowhere in the plant world. Fungal proteins look more like animal than plant proteins. And then there are sterols - important alcohol groups that play a part in everything from biological messenger systems to cell walls. For a long time, these were considered the Achilles Heel of the fungi-animal connection, because while animals have cholesterol, fungi have ergesterol. How could they be alike if they differ in such a fundamental way? Then it was discovered that both animals and fungi contained a component called lanosterol, while plants had nothing like it. The weakest link turned out to be the strongest argument.

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Obviously, no one's committing cannibalism when they eat a mushroom risotto. (Unless human meat is also in the risotto, but that's beyond the scope of this article.) Still, it's clear that whatever the common ancestor between humans and mushrooms was, it was closer to us than it was to plants. The flavor we're savoring in that risotto has more in common with us than it does with the rice. Maybe that's why we like it so much.